A police support officer guards a property in Pimlico, in central London August 26, 2010 Credit: Reuters

Mr Williams, of Anglesey, North Wales, was found in a large North Face holdall, sealed by a padlock, at his top-floor flat in Alderney Street.

A battery of post-mortem tests failed to determine how he died and police originally found it would have been impossible for him to have locked himself inside.

Family lawyer Anthony O'Toole has said the inquest at Westminster Coroner's Court must establish why there was no evidence of another person in his London apartment.

He told a pre-inquest review: "The impression of the family is that the unknown third party was a member of some agency specialising in the dark arts of the secret services, or evidence has been removed post-mortem by experts in those dark arts."

The mathematics prodigy worked as a cipher and codes expert for GCHQ, the Government listening station, but had been on secondment with MI6 since March 2010.

Mr O'Toole added: "In our submission, to properly explore the circumstances of the death, we need to establish the deceased's work."

Relatives want to know why the alarm was not raised when Mr Williams initially failed to turn up to work.

By the time officers arrived at his flat, his body was so decomposed that evidence had been lost.

It emerged last month that two areas of investigation were red herrings.

Forensic teams mistakenly flagged up a spot of DNA on Mr Williams's hand in 2010, before realising just six weeks ago that it matched a scientist on the crime scene.

It also emerged that a Mediterranean couple police wanted to speak to were irrelevant to Mr Williams's death.

Dr Wilcox has indicated she may want to see a practical demonstration of how Mr Williams might have got into the bag and locked it himself.

Experts agree that locking the bag from the inside "would have been very difficult, if not impossible", Metropolitan Police lawyer Vincent Williams said.

The inquest will hear that Mr Williams may have died after breathing too much carbon dioxide.

There were no signs of struggle on his body and blood tests have not shown any drugs in his system.